1. Field of the Invention
This invention relates to information storage and retrieval, and more particularly, to apparatus in a cyclic store control unit for dynamically testing the control unit circuits at a device interface and automatically compensating for accumulative signal propagation delays.
Typical bulk memory subsystems having direct access storage devices such as magnetic disks include a computer driven controller, a disk drive system, a plurality of magnetic recording disks, and speed detection apparatus which may comprise a disk surface having servo timing marks recorded thereon. Other forms of speed detection apparatus such as a digital tachometer may be utilized. The timing marks are sensed and utilized to drive a clock signal generator into synchronization with the sensed timing marks to produce a series of clock signals. The servo derived clock signals are utilized in the store controller for write operations wherein the clock signals may be combined with data to be stored and recorded on one of the magnetic disks as a series of indicia or flux transitions representative of both the clock signals and the data. Data thus recorded with timing inherent therein is termed self-clocking data. The present invention is concerned with circuits for recovering self-clocking data.
During a read or data recovery operation the servo signals from the storage device are ignored and the clock signal generator is synchronized directly with the self-clocking data, which is sensed by a magnetic transducer from a selected storage device.
2. Description of the Prior Art
Many self-clocking modulation and coding techniques are used to improve the efficiency of magnetic-medium devices and to minimize the problems attendant with extremely high bit densities. One such technique is termed modified frequency modulation (MFM). The use of MFM coding derives advantages over other types of codes such as diphase or phase encoding because the MFM code results in fewer flux transitions to represent the same data pattern. Recovered MFM encoded data exhibits an inherent phenomenon called random peak shift (in addition to a predictable peak shift) wherein the recovered data shifts in time by random amounts and direction. Mechanical and electronic design tolerances are critical at high bit densities; minute anomalies such as slight variations in speed of the magnetic medium, asymmetric read-head windings and non-uniform write-current waveform rise times, all contribute to the randomness of the peak shift or jitter. Accordingly, the prior art systems for recovering MFM encoded data required complex circuits with precision components to detect and correct the random peak shift, thereby reducing or eliminating the advantages gained.
Circuits for recovering self-clocking data are designed largely with commercially available integrated circuit modules such as the well known dual-in-line package (DIP). The modules contain circuits and logic elements having variable signal propagation delays, i.e., each delay through each circuit, although within guaranteed maxima and minima, may vary from circuit to circuit and from module to module. Accordingly, it has been the practice in the past to test and select specific modules for individual timing accuracy so that a number of interconnected modules will have more closely predictable signal propagation times. This is an expensive and time consuming procedure. Other prior art data recovery circuits utilize less precise modules with manually adjustable components for compensating for variations in signal propagation time. This technique requires removing the equipment from service periodically for testing and readjustment of the data recovery circuits by a service technician, utilizing a test data string recorded on a magnetic disk. In order to test the ability of data recovery circuits to compensate for random jitter without introducing additional error into the data stream, it is necessary to provide a test data string having no jitter or a known amount of jitter. Such test data strings have been provided in the past from special disks recorded with a high degree of precision. For example, the test data string recorded on the special disk may have a pattern of bits bearing a timed relationship to an ideal bit pattern wherein certain of the bits appear early or late by a known amount in relation to the ideal pattern. Such test disks are expensive to produce, and moreover, the method is device dependent, failing to eliminate errors which might be introduced by the disk drive.